Metallic conductors such as buried pipelines, offshore platforms, etc., require cathodic protection against corrosion. Impressed current cathodic protection systems are widely used, and different such systems have been described in the literature. Such impressed current systems will provide the desired cathodic protection, provided that the cathodic protection current remains above a predetermined value. The cathodic protection current will vary because the resistance in the cathodic protection circuit normally changes. For example, in connection with buried pipelines, the cathodic protection current is impressed onto electrodes buried in a ground bed site. The resistance of the cathodic protection circuit will depend on the effective soil resistance, on other underground metallic structures located in the vicinity of the buried pipeline, on the electric insulation of the various current carrying conductors, etc.
The cathodic protection current can also change as a result of changes in the voltage of the primary source of power. Defects in the insulation of the wiring and welded connections will cause current to be discharged through the insulation and other imperfections. Also, in many regions of the world wide seasonal variations occur in the pipe-to-soil resistivity. Even the buried anodes in the ground bed site deteriorate with age requiring additional cathodic protection current.
From the above it will be apparent that both the voltage and the current in the cathodic protection circuit can vary, and an appreciable drop in either is likely to cause corrosion in the pipeline or other conductor being cathodically protected. Moreover, storm activity or other hazards can cause a breakdown in the commonly used rectifiers in cathodic protection circuits, thereby stopping the flow of cathodic protection current entirely.
Relay circuits have been provided for detecting a drop in the impressed cathodic protection current flowing to a conductor being protected against corrosion. However, such relay circuits that are known have been generally unreliable in that they were difficult to regulate, relatively insensitive to minor changes in cathodic protection current flow, and required a substantial voltage drop before they could provide the desired alert signal to indicate an abnormality.